Grippers are widely used to grasp and transport objects, for example in robotics, teleoperation (master-slave control), and prosthetic devices. It is desirable to provide a gripper that will facilitate stable grasping.
One way to measure the stability of a grasp configuration is by the number of contacts between the object and the gripping jaws. Grasp configurations with exactly 2 edge contacts are referred to as wedged and grasp configurations with 3 or more edge contacts are referred to as stable. For example, consider grasping a hex nut with a parallel-jaw gripper. The hex nut is in a wedged configuration when it is cocked along two opposite edges (FIG. 4) and in a stable configuration when it is gripped along two opposite faces (FIG. 6). Stable grasp configurations are generally preferred because the object is less likely to slip out of the gripper when transported or inserted.
If there is low friction between the object and the gripping jaws, the object will tend to rotate into a stable grasp configuration as the jaws are closed. At the same time, it is required that there exists high friction between the object and the gripping jaws to counter the effect of gravity as the object is lifted and carried. Therefore, it is desired to have low friction along one direction and high friction along a second direction.